Sam McQueen admitted Southampton's performance at Sparta Praha was not up to scratch and apologised to the fans who had made the trip to Czech Republic to support the team.
Saints were beaten 1-0 at Generali Arena, after Costa's early strike for the hosts, meaning they need either a goalless draw or a win at home to Hapoel Be'er Sheva in their final group match to progress to the knockout stages of the UEFA Europa League.
"We just want to apologise to the fans that made that journey out here, because it was poor from us," said McQueen, who played the full game at left-back. "But we can still get through and qualify from the group which is what we're aiming for.
"I think we responded well straight after the goal and created a few chances, but it wasn't good enough from us. It's just about being really clinical and we need to start to get back to getting those clean sheets.
"I think we need to be a bit more ruthless and clinical in the final third. It's always being worked on in the training ground. We've got quality up front, so we're confident goals will come, but just tonight it's not good enough from the whole team."